1. COS 461: Computer Networks
Mobility
Mike Freedman
COS 461: Computer Networks

2. Why (and How) Things Move

3. Physical Mobility
Device attaches to a new attachment point

4. Multi-Homing
WiFi 3G
Device starts using a different attachment point

5. Migration
Process or virtual machine migration

6. Failover
Backup machine takes over after the primary fails

7. Handling Mobility

8. Change Address of Mobile Node?
name
logical link A B
address b1 a
path ?

9. Keeping the Address the Sameb1 a
path b1
Mobility is a routing problem
Change the routes to reach the new location
Challenge: scalability of the routing protocol

10. Changing the Address A B Mobility is a directory problem name
logical link A B a b1 b2
Mobility is a directory problem
Change the mapping of name to address
Challenge: scalability of directory, updating neighbor

11. Two Internet Design Decisions
Socket abstraction
Connection between a pair of fixed IP addresses and port numbers
Leads to more emphasis on routing solutions
Interface addresses
Addresses refer to interfaces (adaptors)
Not the host, or the service 3G
WiFi

12. Routing Solutions
Address Stays the Same

13. Three Examples Ethernet IP routing Mobile IP
MAC learning of the new location
IP routing
Inject IP address(es) at new location
Mobile IP
Stationary home agent directs traffic to new location

14. Example #1: Ethernet MAC learning b1 a b1
Learn b1’s location when b1 sends a frame
Soft state: timeout the cached information

15. Making Larger Ethernet Segments
Ethernet handles mobility
IP address and MAC address stay the same
Switches learn to route to the new location
But, larger networks have multiple segments
Cannot retain your IP address as you move
Solution: virtual local area networks (VLAN)
Logical Ethernet segment spanning a campus
E.g., interconnecting the WiFi access points

16. Pros and Cons Advantages Disadvantages
Seamless mobility, no changes to hosts or apps
No changes to MAC or IP addresses
Disadvantages
Ethernet does not scale
Long paths, state per MAC address, flooding, …
Widely used approach in campus networks

17. Example #2: IP Routing
Node has a persistent address (e.g., )
Injected into routing protocol (e.g., OSPF)
/24
/32

18. Boeing Connexion: Wide-Area Mobility
/24
BGP

19. Pros and Cons Advantages Disadvantages
Seamless mobility, no MAC or IP address changes
Traffic follows an efficient path to new location
Disadvantages
Does not scale to large number of mobile hosts
More routing-protocol messages
Larger routing tables to store smaller address blocks 19

20. Example #3: Mobile IP
Home network: permanent “home” of mobile (e.g /24)
Home agent: performs mobility functions on behalf of mobile
wide area network
Permanent address: always be used to reach mobile, e.g.,
Correspondent: wants to communicate with mobile
correspondent

21. Example #3: Mobile IP
Permanent address: remains constant (e.g., )
Visited network:
e.g., /24
Care-of-address: in visited network
(e.g., 79, )
wide area network
Foreign agent: performs mobility functions for mobile
correspondent

22. Example #3: Mobile IP 1
mobile contacts foreign agent on entering visited network 2
foreign agent contacts home agent home: “this mobile is resident in my network”
wide area network

23. Example #3: Mobile IP 3
foreign agent receives packets, forwards to mobile 2
home agent intercepts packets, forwards to foreign agent 4
mobile replies directly to correspondent
wide area network 1
correspondent addresses packets using home address of mobile
correspondent

24. Pros and Cons Advantages Disadvantages
Seamless to the remote end-point
No routing-protocol overhead
Disadvantages
Overhead of running home and foreign agents
Inefficient “triangle routing” (high “stretch”)
Foreign agent sends “spoofed” IP source address 24

25. Questions Between three mobility choices
(A) Ethernet (B) IP Routing (C) Mobile IP (D) All
Which option:
1. Scales to entire Internet
2. Less efficient communication when mobile
3. Seamless to endhosts
4. Mobility solution does not run risk of filtering
1: (C) Mobile IP
(C)Mobile IP
(D) All
(A) Ethernet

26. Change the mapping of name to address
Directory Solutions
Change the mapping of name to address

27. Three Examples Ethernet Dynamic DNS Various recent proposed designs
Gratuitous ARP to change the MAC address associated with an IP address
Dynamic DNS
DNS updates to change the IP address(es) associated with a domain name
Various recent proposed designs
Updating the remote end-point (e.g., end host, edge switch) to use a new address

28. Example #1: Ethernet Backup machine floods “gratuitous ARP” responseIP
MAC m1 IP
MAC m2
Backup machine floods “gratuitous ARP” response
Associates the IP address with a new MAC address
Hosts update their ARP cache

29. Ethernet Multi-HomingIP
MAC m1 IP
MAC m2
Gratuitous ARP
Balance traffic over two interfaces
Fail over from one interface to the other

30. Pros and Cons Advantages Disadvantages Used in data-center networks
Seamless change from one MAC address to another
Disadvantages
Works only within a single Ethernet subnet
Scalability limitations of Ethernet
Used in data-center networks
But doesn’t help with smart phones homed to multiple administrative domains

31. Example #2: Dynamic DNS Dynamically update DNS
Name:
IP:
Name:
IP:
Dynamically update DNS
Change mapping of name to IP address
Future DNS requests get the new address

32. Applications of Dynamic DNS
Replicated services
Direct future requests to a different replica
E.g., for failover, load balancing, performance, etc.
Services on dynamically-assigned IP addresses
Residential user with a dynamic IP address
Directs clients to the server’s current address
“Fast flux” in botnets
Hiding phishing and malware delivery servers
… behind constantly changing IP addresses

33. Pros and Cons Advantages Disadvantages No new infrastructure
Leverages existing DNS servers
Disadvantages
Only helps for new connections
Overheads of updating DNS servers
Stymied by DNS caching

34. Example #3: Updating the End-Points
Mobile node updates the remote end-point
Sends the remote end-point the new IP address
Allowing ongoing connection to continue
Can be used in conjunction with Dynamic DNS

35. Updating the Edge Switches
Update the switches
Hosts retain their addresses
Switches rewrite the addresses, or encapsulate
Used in some data-center networks

36. Pros and Cons Advantages Disadvantages Work in progress
Scalability of hierarchical addressing
Efficiency of routing along short paths
Disadvantages
Changes to the end host (e.g., apps, TCP, etc.)
… or support from the edge switches
Difficulty when both end-points move at once
Work in progress
Used in some data centers, recent standards/projects
E.g. Princeton’s Serval project (

37. Mobility Today Limited network support for mobility
E.g., within a single Ethernet subnet
E.g., among base stations on a campus
Applications increasingly robust to mobility
Robust to changes in IP address, and disconnections
E.g., client contacting the server, and allowing reading/writing while disconnected 37

38. Mobility Tomorrow Increasing demand for seamless IP mobility
E.g., continue a VoIP call while on the train
E.g., virtual machine migration within and between data centers
Increasing integration of WiFi and cellular
E.g., multi-homed cell phones that can use both networks (announced for Samsung Galaxy S5)
E.g., servers with multiple interface cards
Need better mobility & multi-homing solutions!